1. Field of the Invention
The present invention relates to a method for the purification and/or electrolysis of an aqueous potassium chloride solution.
2. Discussion of the Background
The electrolysis of potassium chloride involves the use of an aqueous potassium chloride solution of high purity. To prepare the potassium chloride solution (the so-called "raw brine"), potassium chloride salt (KCl) is dissolved in water, and is then purified. The impurities introduced into the raw brine by the potassium chloride salt include calcium ions, magnesium ions, iron ions and sulphate ions, which are typically removed in precipitation and filtration processes. This typically involves adding solutions of precipitants (reagents which cause the precipitation of impurities) to the raw brine main stream or a side stream, either before or immediately after the saturation process. Magnesium and iron are precipitated as hydroxides by the addition of caustic potash solution, and calcium is precipitated as calcium carbonate by the addition of potassium carbonate.
Elevated sulphate ion concentrations (for example, &gt;10 g/l of SO.sub.4.sup.2-) in the electrolysis process lead to an increase in the oxygen concentration of the the chlorine produced by electrolysis, the removal of sulphate ions from the raw brine is particularly important.
It is known to remove sulphate ions from the raw brine by precipitation as CaSO.sub.4 or CaSO.sub.4. 2 H.sub.2 O. According to German Patent No. 12 09 562, from an alkali metal chloride brine, by addition of CaO or Ca(OH).sub.2 and Na.sub.2 CO.sub.3 in a 2-step process, magnesium is precipitated as magnesium hydroxide, sulphate ions are precipitated as CaSO.sub.4 or CaSO.sub.4. 2H.sub.2 O and the remaining contaminants are precipitated as carbonates.
German Patent No. 38 05 266 teaches a method for removing sulphate from alkali metal chloride brine by suspending and slaking solid CaO at a pH of 2-9 to a side stream, thus precipitating sulphate ions as CaSO.sub.4.2H.sub.2 O.
However, precipitation of sulphate ions as CaSO.sub.4 or CaSO.sub.4.2H.sub.2 O requires the use of precipitation chemicals (e.g., CaCl.sub.2, Ca(OH).sub.2, etc.), thus increasing the amount of non-recoverable waste mud which has to be tipped or discarded. Moreover, the concentration of Ca.sup.++ ions in the raw KCl brine is increased, which interferes with the electrolysis process.
A very frequently employed method for removing sulphate ions from raw KCl brine involves precipitation as barium sulphate, using barium chloride (BaCl.sub.2) or barium carbonate (BaCO.sub.3) as the precipitant (see "Ullmanns Encyclopadie der Technischen Chemie [Ullmann's Encyclopaedia of Industrial Chemistry]," Vol. 9, 4th ed., p. 339). This method, however, has the following drawbacks:
(1) The barium salts BaCl.sub.2 and BaCO.sub.3 are toxic, so that handling them requires special safety measures. PA1 (2) Barium salts are expensive. PA1 (3) The filter sludge generated and filtered off contains, in addition to barium sulphate, residues of undissolved barium chloride or carbonate. PA1 (4) Inevitably, barium ions will be retained in the KCl brine after the filtration process. BaSO.sub.4 precipitates are thus inevitably formed on the activated titanium anodes used in the electrolysis cell. These precipitates lead to an increase in the cell voltage required to electrolyze the KCl brine at a given current density, and thus, to increased energy costs. PA1 (5) The operating lifetime of the titanium anodes is shortened, which results in decreased productivity (yield of product per unit time) and increased reactivation costs.